Boosted engines operating at heavy loads and low engine speeds may be prone to pre-ignition combustion events. The early combustion due to pre-ignition can cause very high in-cylinder pressures, and can result in combustion pressure waves similar to combustion knock, but with larger intensity. Strategies have been developed for prediction and/or early detection of pre-ignition based on engine operating conditions. Additionally, following detection, various pre-ignition mitigating steps may be taken.
In one example, as shown by Rollinger et al. in U.S. Pat. No. 8,073,613, in response to a pre-ignition event, the affected cylinder is enriched. In addition, an engine load may be reduced. Further still, the enrichment and load limiting is adjusted based on the pre-ignition history of the engine with a more aggressive enrichment and load limiting applied when there is recurrent pre-ignition as compared to sporadic pre-ignition.
However, the inventors herein have identified a potential issue with such an approach. Specifically, the enrichment is deactivated in response to a tip-out leading to an incomplete pre-ignition mitigating enrichment. For example, if an engine is operated at a relatively higher engine speed and load condition, the engine can become very hot. Subsequent operation at lower engine speed and high load can produce repeated pre-ignition events every time a throttle tip-in is performed to increase air mass and torque. The approach of '613 mitigates low speed pre-ignition by enriching the cylinder(s) for a short period of time. However, if the pedal or throttle request is decreased (e.g., due to a tip-out event) during the mitigating action, the enrichment strategy may be abandoned. A subsequent tip-in to even low or mid loads may produce pre-ignition events. In particular, pre-ignition may be triggered during a subsequent tip-in to loads even lower than those that typically trigger pre-ignition. This may not only degrade engine performance but also reduce engine life.
To at least partly address the above issue, a method of controlling an engine is provided. The method comprises, during a tip-in, in response to an indication of pre-ignition, enriching the engine until a subsequent tip-out, and if a number of enrichment cycles between the tip-in and the tip-out is lower than a threshold, enriching the engine during a subsequent tip-in. In this way, enrichment responsive to pre-ignition induced during a tip-in may be completed and further pre-ignition may be mitigated.
For example, in response to an indication of pre-ignition during a tip-in event, an enrichment profile may be determined. This may include a number of enrichment cycles as well as a degree of richness of each cycle, to be performed to mitigate the pre-ignition. If a tip-out event occurs, the enrichment may be stopped or deactivated. If the tip-out occurs after the number of enrichment cycles are performed, the enrichment profile may be considered completed. However, if the tip-out occurs before the determined number of enrichment cycles are performed, the enrichment profile may be considered incomplete. The controller may then store the remaining number of enrichment cycles in its memory. During a subsequent tip-in following the tip-out, the remaining number of enrichment cycles may be executed, even if an indication of pre-ignition is not received during the subsequent tip-in. This allows the enrichment initiated responsive to the initial tip-in to be completed or reactivated during the subsequent tip-in, and further pre-ignition events to be preemptively addressed.
Alternatively, the controller may lower a pre-ignition threshold during the subsequent tip-in and perform the remainder of the enrichment when the engine conditions cross the lower threshold. For example, the remainder of the enrichment may be triggered during the subsequent tip-in at lower engine load conditions than engine load conditions where pre-ignition typically occurs. The controller may then complete the enrichment until the remaining number of enrichment cycles have been depleted.
In this way, pre-ignition responsive to repeated tip-ins may be reduced. By continuing enrichment initiated, but not completed, during an initial tip-in, at a subsequent tip-in, sufficient combustion chamber cooling can be provided. By preemptively reactivating the enrichment during the subsequent tip-in, before an indication of pre-ignition is received, the combustion surfaces can be maintained below critical temperatures and thermal overloading during the subsequent tip-in can be reduced. In this way, not only is the incipient pre-ignition event mitigated, but also, the likelihood of further pre-ignition events is reduced. Overall engine performance and life is extended.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.